For example, in an approximately cylindrical-shaped barrel portion of a steam generator in a pressurized-water nuclear power plant, a heat transfer tube group formed of a number of heat transfer tubes that are bent in a U-shape and whose ends are secured to a tube plate is provided in a state where it is covered by a tube-group outer casing.
Water (supply water) supplied to an upper portion of an annular channel formed between the inner wall of the barrel portion and the tube-group outer casing flows down through the annular channel, flows into the tube-group outer casing from a lower portion of the annular channel, and rises along the heat transfer tubes. During this process, since pressurized high-temperature coolant from a reactor flows through the heat transfer tubes and heats the heat transfer tubes, the water that is in contact with the outer surfaces of the heat transfer tubes is heated and moves upward while evaporating.
By means of a steam/water separator provided above the heat transfer tube group, the water is separated by into steam and hot water, and the hot water is returned to the annular channel whereas the steam is sent, for example, to a turbine in a secondary system after moisture contained in the steam is separated therefrom.
Since the supply water takes heat from the high-temperature coolant flowing through the heat transfer tubes, the temperature of the coolant is gradually reduced from an inlet side toward an outlet side. In this way, since the coolant in the heat transfer tubes has a relative temperature difference, an area located from the inlets of the heat transfer tubes to the apexes of the U-shape thereof may also be referred to as a hot side and an area located from the apexes to the outlets thereof may also be referred to as a cold side.
In this steam generator, since the supply water is mixed in the entire area of a lower portion of the tube-group outer casing, the temperature of the supply water in this portion is made similar, thus reducing the temperature difference between the coolant in the outlets of the heat transfer tubes and the supply water. Therefore, since the amount of heat exchange is reduced in this portion, the total amount of heat exchange of the steam generator is reduced.
Various ways of improving this point have been proposed, and, for example, Patent Citations 1 and 2 describe examples thereof.
In the examples, supply water is supplied only to a cold-side portion of an annular channel to reduce the temperature of the supply water flowing into the cold side. This increases the temperature difference between the coolant in the outlets of the heat transfer tubes and the supply water to increase the amount of heat exchange in this portion.
Patent Citation 1:
Japanese Unexamined Patent Application, Publication No. Hei-03-87501
Patent Citation 2:
Japanese Unexamined Patent Application, Publication No. 2000-9888